In a vertical cavity surface emitting laser (VCSEL), the lasing wavelength is determined by the length of a Fabry-Perot cavity formed by two distributed Bragg reflectors (DBRs) separated by a semiconductor cavity that includes layers with optical gain. The optical gain in a VCSEL is typically provided by quantum wells. Each quantum well has a gain spectrum with a single peak wavelength, and some spectral width over which gain is present. Each DBR is composed of quarter wave layers of alternating high and low refractive indices. The DBR reflectivity is characterized by a complex amplitude and phase spectrum. The amplitude spectrum exhibits a high reflectivity region at the center of which the reflectivity is highest. The width of the high reflectivity region is referred to as the DBR stop-band width. The phase characteristic of the DBR varies approximately linearly over the stop-band width. The lasing wavelength of a VCSEL is determined by the optical length of the semiconductor cavity and the phase characteristics of the DBRs. The gain provided by the active layer, necessary to achieve lasing (threshold gain) is determined by the round-trip cavity loss that includes material absorption and the DBR transmission. A monolithic multiple-wavelength VCSEL array requires side-by-side fabrication of VCSELs with varying lasing wavelengths, but otherwise uniform laser characteristics: threshold gain and current, and efficiency. This implies that the vertical structure of the lasers must vary from device to device on the same wafer, while the cavity losses, material gain, and the DBR transmission remain largely unchanged. The lasing wavelength variation is most commonly realized by changing the optical length of the semiconductor cavity.
One prior art method to making a monolithic multiple wavelength VCSEL array is non-uniform growth due to thermal gradient. The backside of a substrate is patterned prior to epitaxial growth in an MBE reactor. The resulting backside pattern produces a thermal gradient on the surface of the substrate when the wafer is heated. Because growth rate is temperature dependent, there is a variable material thickness and hence a variable laser wavelength along the thermal gradient. One disadvantage of this approach is the fact that the arrays are limited to linear geometries. To date, it has been difficult to control the wavelengths precisely and repeatedly over large areas of the wafer.
An alternate prior art method is laterally confining each laser prior to epitaxial growth by either etching a mesa or patterning windows in an oxide mask. This process is known as "selective area growth". Growth rate and composition are functions of lateral dimension. The method is sensitive to growth conditions and may vary from reactor to reactor or from growth to growth. In both of the aforementioned prior art methods, the proximity of different wavelength devices in an array is limited.
Another prior art method is to grow a partial VCSEL structure including the lower DBR, the active region, and some part of the upper DBR. The wafer is masked and anodically oxidized to some controlled oxide thickness over the exposed portions. A selective etch is then used to remove the oxide. This process is repeated to create different effective cavity lengths for each laser in an array. The remainder of the VCSEL structure is regrown over the patterned wafer. Each etch is sensitive to voltage and concentration variations that may affect the depth, resulting in reduced control over wavelength spacing between devices.
A method that is insensitive to fabrication process variations, and has the same accuracy as the planar epitaxial growth used to fabricate the laser mirrors and the active layer, is preferred for fabrication of multiple-wavelength VCSEL arrays. Such a method should also allow for design flexibility in device spacing and geometry within the array.